Unraveling the Physical Origins of Photoacidity and Photobasicity: Charge Transfer and Relief of Excited-State Antiaromaticity

While photoacids and photobases are known to experience an enhanced thermodynamic driving force for proton transfer upon electronic excitation, the underlying physical origin of this phenomenon is not fully understood. Here, we describe our efforts to develop descriptors for photoacidity and photobasicity based on intramolecular charge transfer, excited-state antiaromaticity relief, and an energy decomposition analysis (EDA) of their hydrogen-bonding complexes with H₂O. We demonstrate that intramolecular charge-transfer descriptors are generally less effective than those involving excited-state antiaromaticity relief or hydrogen-bonding EDA. Overall, our work provides new insights into the properties of known photoacids and photobases, while also supporting efforts to identify novel photoacids and photobases with an increased thermodynamic driving force for excited-state proton transfer.